Problem with Grease Pencil Keyframes for Colors

I’m working on a simple grease pencil animation on blender 2.8. Now for the animation I want to have a part of my Grease Pencil object fade.
First time i tried it worked with just keyframing the opacity of the layers i wanted to fade in my GP Object. But as I wasn’t happy i deleted the keyframes for the opacity again to change them a little bit. Here my problem started: I can still ad one keyframe with a set opacity ( 0.5 for example) but as soon as I try to add a second keyframe with a different opacity it wont let me insert it. It just keeps jumping back to 0.5 in this case.
I tried out a few things and this problem always seems to occure as soon as i delete a set of keyframes for Materials, Colors or Opacity on a Grease Pencil Object. It all works just fine until then.
The opacity slider has e greenish tint as soon as it starts having this problem (when you are on the exavt keyframe it’s a yellowish tint, pics attached).


here the .blend https://1drv.ms/f/s!AlMGSqnU5vn7owR9C9vasqYw_zEr
Does anyone have the same problem or knows what the problem could be?